1. Field of the Invention
The invention relates generally to methods of and apparatus for drying and heating materials in a stream of hot gases. More particularly the invention relates to methods and apparatus for controlling a material drying and heating process independently of an adjustment of the hot gas generator.
2. Discussion of the Prior Art
Asphalt production facilities typically use drum drying and mixing apparatus for heating aggregate materials and for mixing the materials with liquid asphalt. The drying and heating process of the materials is effected by a flow of heated gases through the length of the drum. Drum drying and drum drying and mixing apparatus is typically characterized as either parallel flow or as counterflow apparatus. In the counterflow apparatus, the direction of flow of the gases is opposite the general flow of the material through the drum. In the parallel flow the gases and material exit at the same end of the drum. In both types of apparatus, a cylindrical drum of substantial size is disposed with its axis in a substantially horizontal position. A slight incline from one end to the other of the longitudinal axis places the end of the drum at which materials are fed into the drum at a slightly higher elevation than the opposite, material discharge end of the drum.
The drum is supported on a frame on trunnion rollers to rotate about its longitudinal axis. Flights are attached to the inner surface of the drum to lift the material as a result of the rotational movement of the drum, and to release the material gradually across an upper arc of the rotational movement of the drum, creating a curtain or veil of falling material within the inner space of the drum. The rate of axial advance of the material within the drum is of course affected by the angle of incline of the axis of the drum and the type of flights within the drum.
The hot gases exiting from the drum contain a substantial amount of water vapor as a result of the drying operation. The exhausting gases further contain fine dust which became entrained in the gas flow. When the operation involves a downstream mixing operation, the gases would typically contain also hydrocarbons in vapor form. The hydrocarbons as vapor are typically undesirable pollutants in that they would condense to form fine droplets of pollution carried by the gas stream when the gases are exhausted directly into the atmosphere.
In compliance with environmental standards, the dust and other pollutants are sought to be removed from the stream of hot drying gases before the gases are returned to the environmental atmosphere. Various types of filters and scrubbing methods are known and have been tried. A popular filtering process involves a filter which is known as a baghouse filter. A baghouse filter is a chamber in which a great number of filter bags are suspended. The hot gases are introduced into the chamber externally of the bags. The bags have upper openings which are coupled to exit ports from which the hot gases can exit to the environment. The gases consequently pass from the outside of the bags through the walls of the bags into the bags and from there to the environment. Particulate material in solid and liquid form is consequently deposited on the outside of the bags as the hot gases exit through the bags from the chamber. As the material deposits and cakes on the surface of the bags, the gas flow through the bags becomes more and more restricted. Baghouse filters consequently include provisions for applying reverse flow pressure to the bags to remove the deposits from the bags. Thus from time to time, all or a selective number of the bags may be "puffed" with air pressure applied to the inside of the bags to cause the caked deposits to drop off from the outside of bags and fall to the base of the chamber. A chute or conveyor in the base of the chamber typically removes the fines deposits from the base of the chamber.
Problems have occurred with respect to cleaning baghouse filters when the temperature of the hot exhaust gases drops below a level at which water vapor begins to condensate. Condensation is likely to occur first across the interface of the bags, possibly because the filtering process is also accompanied by a slight pressure drop across the interface. Such a pressure drop might even increase, as the filter becomes more clogged. Typically, the temperature of the exhaust gases would be measured at the exit chamber from the drum, though the temperature of the gases may decrease further in ducting routing the gases to the filter chamber. For typical installations it has been observed that when the temperature of the exhaust gases drop below a temperature of 250 degrees Fahrenheit, with some variation depending of course on the amount of moisture contained in the exhaust, the material being deposited on the filter walls will tend to become sludgy. The sludge remains pliable and adheres strongly to the walls of the filter bags. As a result, the reverse pressure application to the bags often fails to clear the caked material and renders the baghouse frequently inoperative and ready for extensive downtime.
Problems can also occur when the exhaust temperature exceeds a certain desirable range above the referred to minimum temperature of 250 degrees Fahrenheit. These problems relate to pollution control and possible damage to the filters. Excessive temperatures clearly can damage the filter bags. But also, in order to meet clean air standards, it is desirable to maintain the exhaust temperatures as low as possible above the condensation temperature of water vapor, to allow hydrocarbons to condense and be collected by the fine dust carried by the gases to the filter. Ideal exhaust temperatures would allow any and all hydrocarbons to be condensed and collected by the aggregate fines at the filter walls with substantially no water vapor condensation. It is consequently desirable to control the exhaust temperature of the drum drier apparatus to remain within a narrow range of about 250 degrees Fahrenheit.
The drying process may be regulated by controlling the burner unit, many typical burner units providing a ten-to-one turndown ratio to adjust the burner output to the rate of material flow through the drum such that the material has a desired dryness and temperature at the end of the drying and heating section of the drum. Certain material flow patterns through the drum in the past have caused problems in that a correct dryness and temperature of the aggregate at the end of the drying and heating section of the drum has resulted in an exhaust gas temperature which falls below the desired temperature range for routing the exhaust gases to the baghouse filter. Increasing the burner capacity, however, would have tended to result in an aggregate temperature which may be higher than desired. Additions of recycle material to virging aggregate material add further complexity. Any change in material mixes in various proportions between recycle and virgin aggregate material, and changes in the moisture contents and porosity of the virgin material to be dried are major factors that may cause wide variations in exhaust gas temperatures by affecting heat transfer between the burner-generated hot gases and the aggregate. A change in the mix of virgin material to recycle material typically calls for a change in the final temperature of the virgin material. In many state of the art operations, the heat stored in the virgin material is typically used to dry and heat the recycle material. Thus, at one extreme, with no recycle material to be dried, virgin material would be dried and heated to substantially the desired temperature of the final asphalt mix. At the other extreme, however, with a one-to-one mix ratio of virgin and recycle material, the temperature of the virgin material may be heated well above the desired temperatures of the final mix, in that the heat stored in the virgin material is transferred to the recycle material in an indirect drying and heating operation. In seeking to arrive at the proper asphalt mix temperature, changes in the exhaust gas temperatures continue to present problems.
Consequently, controlling the exhaust gas temperature independently of a burner adjustment continues to be a problem a solution to which would be desirable.